The present invention relates to imaging systems of the type wherein a latent charge image is formed on an imaging member, the latent charge image is developed by a toner, and the developed image is transferred to a receiving member to form a permanent image. Numerous systems of this type exist in the prior art, wherein the latent image is formed by optical or electrical means, and the pigmented toner is a liquid toner or a dry powder toner. The present invention specifically relates to liquid-toned systems.
Any imaging system is subject to broad limitations that affect system performance.
Suspension of the pigment particles in a liquid carrier allows a high degree of process uniformity, and permits the use of very fine toner particles, so that extremely faithful images may be produced when specialized processors or recording sheets permit operation, generally at relatively low speeds, without squeegee or pressurized wet image transfer steps. On the other hand, liquid-toned images may become blurred or distorted during transfer, and may also require special coated papers so as not to soak into, or through, the receiving member. Undesirable wicking along paper fibers may degrade the final image, and environmental concerns are raised by the presence of vapors from the toner carrier, which is generally transferred to the imaging member and partially removed during a fusing step.
Dry toners on the other hand are convenient to handle, and are essentially free of vapor emissions, but they present other limitations related to their development mechanics. The use of generally larger toner particles in dry toners is necessary to limit environmental dust, but can give dry-toned images of low density a grainy appearance; and the mechanical application by cascade or a brush rotating along the sheet feed direction may give rise to small directional artifacts, such as streamers, or background fogging in the final image. Furthermore, development efficiency can become extremely variable as the components of a multi-part developer vary, or as weather conditions that affect charging of the member or transfer of the toner, change.
As demands for greater speed or resolution, or decreased environmental impact, are placed upon such imaging machines, each kind of process is increasingly challenged, and no single design can be expected to simultaneously optimize operating cost, cleanliness, resolution, speed, mechanical simplicity and component lifetime.
Among the greatest problems in such imaging mechanisms are those of forming a toned image, keeping it stable, and transferring this image to the ultimate print sheet (or fixing it on the sheet, in those specialized systems in which the ultimate sheet is directly developed) with speed and good image quality.
In a liquid toned system, transfer to the final sheet may be accomplished by direct contact, in which the liquid and toner particles are simply wicked into the partially absorbent receiving surface. In a powder toned system, the powder-developed image may be transferred by a high pressure nip, or may be transferred by providing electrostatic field at the nip or gap with an imaging roller. Often, specialized intermediate transfer belts or drums are used to pick up the toned image from the latent imaging member and then release it to a recording sheet. Fusing of the transferred image may be accomplished later by applying heat, pressure or both.
Some systems extend across different ones of the above categories. For example, commonly-owned U.S. Pat. Nos. 5,012,291 and 5,103,263 show a belt system wherein a powdered toner is applied to the imaging belt and then brought to a high temperature or even liquefied state on the belt before being brought into contact with a receiving sheet. Commonly-owned U.S. Pat. No. 5,414,498 shows a similar system in which a liquid-toned image is heated on the imaging belt to drive off carrier and change state to a dry image before transfer. U.S. Pat. No. 4,708,460 shows a system where a liquid-toned image is transferred to an intermediate belt 34 that carries it through a heater station, partially vaporizing the carrier and softening the toner particles before the liquid image is transferred and fused at a hot pressure nip, where substantially all the remaining carrier liquid is vaporized. A somewhat similar system intended for multicolor printing is shown in U.S. Pat. No. 4,690,539, wherein liquid images of successive colors are transferred to an intermediate belt on which the carrier from each color step is removed by a vacuum system, thus stabilizing the toners on the belt before transferring the dried toner images to a copy sheet. In each of these latter systems, some or all of the carrier is removed before the image is transferred to a final recording sheet.
U.S. Pat. No. 5,106,710 shows a system wherein one or more liquid toner images are applied to a dielectric-coated paper with a thin release coating. The toned image passes a vacuum squeegee, and is air dried after which the toner image is transferred to a receiving sheet in a heated roller nip or on a hot platen in a vacuum draw-down frame.
Other processes have been proposed several decades ago wherein liquid toners are partially or fully dried, as part of a multicolor liquid-toned process, on an imaging member, and recently very specific systems have evolved, such as the one shown in international application WO91/03006 wherein an intermediate roller member is used to pick up a liquidtoned image, heat it and transfer to a recording sheet. Effective transfer in such systems may depend on the temperatures and surface properties of each of the various sheets and rollers, as well as the viscosity of the image at its transfer nip.